An expansion-molded article using a polypropylene resin composition is used as an interior trim material such as an automotive door trim (see Patent Literature 1).
In addition, an expansion-molded article described above is obtained by: filling a polypropylene resin composition including a chemical expanding agent in a mold which is in a closed state; then cooling the resin composition with an uncooled portion (expandable part) being left in a central portion in the thickness direction; pulling back the core of a movable mold to cause expansion of the uncooled portion (expandable part); and then cooling and solidifying the resin composition.
However, when a conventional polypropylene resin composition is used and an expansion rate is more than two times, cells in an expansion layer existing inside are destroyed, resins at a cell wall are attached to each other and become stringy, a resin becomes stringy, and a void like a cave is generated (see FIGS. 11 and 12), causing: (1) deterioration in low-temperature impact strength; (2) deterioration in surface smoothness of the skin layer; (3) deformation after molding; and the like. Moreover, when content of an expanding agent is increased in order to enhance an expansion rate, a swirl mark is generated at the surface of a molded article, causing poor appearance.
Therefore, only approximately 1.6 times can be obtained as an injection expansion-molded article, which has excellent appearance quality and sufficient low-temperature impact strength, and sufficient weight saving has not been realized yet.
Patent Literature 1: Japanese Published Unexamined Patent Application No. H11-179752